1. Field of the Invention
This invention generally relates to magnetic bubble domain devices and more particularly to a garnet film having a uniaxial magnetic anisotropy for a magnetic bubble domain device being suitable for use as a film for sustaining magnetic bubbles.
2. Description of the Prior Art
As well known in the art, magnetic bubble domain devices have been highlighted as versatile information processing devices, especially memory devices, and active development has been directed thereto.
In applying magnetic bubble domain devices to memory devices, it is necessary to take into consideration the fact that the diameter (d) of a magnetic bubble determines the memory density which is the most important factor of memory function.
Today, the magnetic bubble having a diameter of 3 to 5 .mu.m is practically used but it is expected that the memory density can be highly populated with a small magnetic bubble having a diameter of less than 2.5 .mu.m.
In other words, in order that the magnetic bubble domain devices play the part of the other memory devices, such as disk memories and semiconductor memories, which are typically available at the present time and they are put into practice as memory devices, it is necessary to increase the memory density drastically by decreasing the diameter of the magnetic bubble to the order of less than 2.5 .mu.m. Therefore, there is an ardent demand for advent of a garnet film in which magnetic bubbles of small diameter can exist stably and can propagate.
However, a garnet film for so-called small magnetic bubbles of a diameter of the order of less than 2.5 .mu.m has, as experienced in the art, a bubble collapse field Ho which has great dependencies on temperatures.
For example, a (YSmLu).sub.3 (FeGa).sub.5 O.sub.12 film capable of sustaining magnetic bubbles of a diameter of about 2 .mu.m has a temperature variation rate or coefficient of -0.30 to -0.35%/.degree.C. in respect of Ho at 30.degree. C.
On the other hand, the bias field from a bias magnet usually made of barium ferrite has a temperature variation rate of -0.20%/.degree.C., greatly differing from that of the bubble collapse field.
The great difference in temperature variation rate between Ho of the garnet film for sustaining the magnetic bubbles and the bias field narrows greatly the temperature range which permits stable movement of the magnetic bubbles, thus affecting the magnetic bubble domain device adversely.
Temperature characteristics of a garnet film for a magnet bubble domain device are discussed, for example, in the following references:
(1) "Temperature Variation of Magnetic Bubble Garnet Film Parameters" By R. M. Sandfort et al, AIP Conf. Proc. 18, (1), pp 237-241 (1973);
(2) "Growth Reproducibility and Temperature Dependencies of the Static Properties of YSmLuCaFeGe Garnet" by G. G. Sumner et al, AIP Conf. Proc. 34, pp 157-159 (1976); and
(3) "Properties of Gd.sub.y Y.sub.3-y Fe.sub.5-x Ga.sub.x O.sub.12 Films Grown by LPE" by Jerry W. Meody et al, IEEE transactions on magnetics, Vol. Mag-9, p. 377 (1973).
Reference (1) describes temperature characteristics of a garnet film for magnetic bubbles but discloses neither the improvement in temperature characteristics of a garnet film for small magnetic bubbles in respect of Ho nor a composition of the present invention.
Reference (2) discloses a YSmLuCaFeGe garnet as a film material having a temperature coefficient of -0.2%/.degree.C. in respect of Ho. This film is, however, of a Ca-Ge system garnet which is totally different from the composition of the present invention. In addition, Ho cannot be controllable so that it is impossible to determine the value of Ho which fairly matches with the bias field used.
A garnet film disclosed in reference (3) and containing Gd and Ga is totally different from the composition of the present invention because quantities of Gd and Ga are different from those of the present invention and Sm and Lu are excluded. In addition, this garnet film is unsuitable for small magnetic bubbles and Ho is not referred to in reference (3).